At present, several methods for the transmission of the message traffic between the unit computers of the network elements of packet-switched telecommunication systems are known. For the implementation of the packet-switched telecommunication system itself, one method is to use the ATM technique (Asynchronous Transfer Mode, ATM). Another example of packet-switched methods is the Frame Relay technique.
ATM is a connection oriented packet-switched data transfer method which is characterized by the use of standard-length cells for data transfer. Each cell consists of a 5-byte header and a 48-byte information part. The header fields comprise a virtual path identifier (VPI) and a virtual channel identifier (VCI), a payload type identifier (PTI), a cell loss priority bit (CLP) and a header error control (HEC) which can be used to correct one-bit errors and to detect two-bit errors. In an ATM switch, the cells are transferred from a logical input channel to one or more logical output channels. A logical channel consists of the number of the physical link, such as e.g. an optic fiber, and the identifier of the channel in this link, i.e. the VPI/VCI data. A single physical transmission medium, such as an optic fiber, may contain a plurality of virtual paths VP, and each virtual path may contain a plurality of virtual channels VC.
Since the cells have a standard length, the switching in the ATM switches can be performed on the basis of the cell header at hardware level and therefore very fast. Cells belonging to different connections are distinguished from each other by the virtual path and virtual channel identifiers. To set up a connection over the network, a fixed route, i.e. a virtual link is defined, and the cells comprised in the connection are routed along this virtual link. In the nodes of the network, the cells are connected on the basis of the VPI/VCI values. The VPI/VCI values of the cells are defined separately for each leg of the connection, so they generally change in conjunction with the connection of VP-level or VC-level. At the end of the data transfer, the link is disconnected.
The ATM protocol is described using an ATM protocol model, which is a layered model resembling the OSI (Open Standards Interconnection) model. The topmost level in the model is the data coming from the user. The next layer below it is an ATM adaptation layer (AAL). Below that is again an ATM layer, below which there is further a physical layer (PHY). The AAL layer is divided into two parts, a SAR layer (Segmentation And Reassembly) and a CS layer (Convergence Sublayer). The CS layer is further divided into two sublayers: SSCS layer (Service Specific Convergence Sublayer) and CPCS layer (Common Part Convergence Sublayer).
The ATM adaptation layer divides the frames of the layers above it into pieces, places the pieces into cells and reassembles the frames at the receiving end.
The ATM layer again offers a cell transfer service to the AAL layer. It only deals with the cell header, taking care of the connection, multiplexing and demultiplexing of cells, generation and deletion of cell header, and generic flow control (GFC) at the user network interface (UNI). In addition, the ATM layer takes care of detection and correction of header errors as well as segment synchronization.
The physical layer is also divided into two sublayers, a PMD (Physical Medium Dependent) sublayer, which takes care of transfer-system specific functions at bit level; and a transmission convergence sublayer (TCS), which takes care of adaptation of cells to each transmission system and definition of cells, error checks on cell headers and cell rate adjustment.
A network element consists of a cross-connection part, which takes care of actual cross-connection of data, and a control part, which performs various control actions. Typically, both the cross-connection part and the control part comprise several unit computers. Message traffic flows between these unit computers.
FIG. 1 presents a prior-art solution for the transmission of message traffic between unit computers 20 in the network elements of packet-switched telecommunication systems. The message traffic between the unit computers Ck of the control part is transmitted via a separate higher-capacity path and the message traffic e.g. for the setup of cross-connections between the unit computers Cr of the cross-connection part and the unit computers Ck of the control part is transmitted via a separate lower-capacity path. In the figure, the dotted line represents a message traffic connection.
Another prior-art solution is presented in FIG. 2. The message traffic between the unit computers Ck of the control part has been routed via the network element's own cross-connections. However, the message traffic between the unit computers Cr of the cross-connection part and the unit computers Ck of the control part still flows via a separate path.
FIG. 3 also presents a prior-art implementation. In this case, the cross-connection is not a universal connection. An example of this type of different cross-connection is to transmit, in systems based on the ATM technique, so-called in-band control cells which flow along with the rest of the traffic and which are removed from the cell flow by the target circuit. Another example of different cross-connection is an implementation in which a unit computer is placed directly on the plug-in unit of the second terminal point of the cross-connection.
However, such solutions have the drawback of poor scalability. The improve the scalability of the message traffic system, projects have been started to develop systems e.g. in ATM networks in which the ATM switch itself is used for the transmission of message traffic, in which case message traffic would be carried in the subscriber traffic flow. In such a system, e.g. in the case of ATM, the computer units are connected to the ATM switching fabric either directly via a separate line card or via an ATM multiplexer. The unit computers of the line cards, i.e. plug-in units, are connected to the ATM switching fabric via the ATM circuits on the cards to allow the transmission of message traffic. This is the fundamental principle behind e.g. the system described in an article entitled The MainStreetXpress Core Services Node—A Versatile ATM Switch Architecture for the Full Service Network, published in IEEE Journal on Selected Areas in Communications, Vol. 15, No. 5, June 1997.
A problem in the prior-art systems is that they are dependent on circuit-specific properties, such as, e.g. in the case of ATM, the cell insertion and deletion functions of the ATM circuit. As a consequence, each plug-in unit type needs a specific type of SAR (Segmentation And Reassembly) interface for connection to the ATM circuits in the plug-in unit in question. A further problem is that the transmission of both subscriber traffic and message traffic via the same lines is difficult to manage.
The object of the invention is to disclose a new type of system and method that will eliminate the problems referred to above. A specific object of the invention is to disclose a flexible and optimal system and method for the transmission of message traffic in packet-switched telecommunication systems.